1. Field of the Invention
This invention generally relates to organic light emitting diode (OLED) displays and, more particularly, to an OLED display fabricated on a thin metal foil substrate.
2. Description of the Related Art
As noted in U.S. Pat. No. 6,392,617 (Robert Gleason), arrays of OLEDs are utilized to create two-dimensional flat panel displays. As compared to conventional light emitting diodes (LEDs), which are made of compound semiconductors, the low cost and ease of patterning OLEDs makes compact, high resolution arrays practical. OLEDs can be adapted to create either monochrome or color displays and the OLEDs are conventionally formed on glass or semiconductor substrates.
As is known in the art, arrays of OLEDs and LEDs are typically classified as passive matrix arrays or active matrix arrays. In a passive matrix array, the current drive circuitry is external to the array, and in an active matrix array the current drive circuitry includes one or more transistors that are formed within each pixel. An advantage of active matrix arrays is that they do not require peak currents that are as high as passive matrices. High peak currents are generally undesirable because they reduce the luminous efficiency of available OLEDs. Because the transparent conducting layer of an active matrix can be a continuous sheet, active matrix arrays also mitigate voltage drop problems that are experienced in the patterned transparent conductors of passive matrices.
FIGS. 1 and 2 are depictions of conventional active matrix pixels (prior art). It should be understood that although individual active matrix pixels are shown for description purposes, the individual active matrix pixels shown in these figures are typically part of an array of pixels that are located closely together in order to form a display. As shown in FIGS. 1 and 2, each of the active matrix pixels includes an address (gate) line 102 and 202, a data line 104 and 204, an address transistor 106 and 206, a drive transistor 108 and 208, a storage node 110 and 210, and an OLED 112 and 212. The address lines allow the pixels to be individually addressed and the data lines provide the voltage to activate the drive transistors. The address transistors control the writing of data from the data lines to the storage nodes. The storage nodes are represented by capacitors, although they need not necessarily be separate components, as the gate capacitance of the drive transistors and the junction capacitance of the address transistors may provide sufficient capacitance for the storage nodes. As shown, the OLEDs are connected to a drive voltage (VLED) and the current that flows through the OLEDs is controlled by the drive transistors. When current is allowed to flow through the drive transistors, the OLEDs give off light referred to as a luminous flux, as indicated by the arrows 114 and 214.
Referring to FIG. 1, PMOS transistors are preferred when the cathode of the OLED 112 is grounded, and referring to FIG. 2, NMOS transistors are preferred when the anode of the OLED 212 is connected to the supply voltage (VLED). Utilizing the PMOS and NMOS transistors as shown in FIGS. 1 and 2 makes the gate to source voltages of the drive transistors 108 and 208 insensitive to voltage drops across the OLEDs, thereby improving the uniformity of the light 114 and 214 that is given off by the OLEDs.
The operation of the prior art active matrix pixels is described with reference to the active matrix pixel configuration shown in FIG. 2, although the same concepts apply to the active matrix pixel of FIG. 1. The active matrix pixel shown in FIG. 2 serves as an analog dynamic memory cell. When the address line 202 is high, the data line 204 sets the voltage on the storage node 210, which includes the gate of the drive transistor 208. When the voltage on the storage node exceeds the threshold voltage of the drive transistor, the drive transistor conducts causing the OLED 212 to emit light 214 until the voltage on the storage node drops below the threshold voltage of the drive transistor, or until the voltage on the storage node is reset through the address transistor 206. The voltage on the storage node will typically drop due to leakage through the junction of the address transistor and through the gate dielectric of the drive transistor. However, with sufficiently low leakage at the address and drive transistors and high capacitance at the storage node, the current through the OLED is held relatively constant until the next voltage is set on the storage node. For example, the voltage is typically reset at a constant refresh interval as is known in the art. The storage node is represented as a capacitor in order to indicate that sufficient charge must be stored on the storage node to account for leakage between refresh intervals. As stated above, the capacitor does not necessarily represent a separate component because other sources of capacitance on the storage node may suffice.
In the active matrix pixel of FIG. 2, the voltage on the storage node 210 determines the intensity of the light 214 that is generated by the OLED 212. If the intensity-current relationship of the OLED and gate voltage-current relationship of the drive transistor 208 are known, according to one method, the desired intensity of light is generated by placing the corresponding voltage on the storage node. Setting the voltage on the storage node is typically accomplished by utilizing a digital to analog converter to establish the voltage on the corresponding data line 204. In an alternative method, the storage node is first discharged by grounding the data line, and then the data line is set to the CMOS supply voltage (Vdd). Utilizing the latter method, the address transistor 202 functions as a current source, charging the storage node until the storage node is isolated by setting the address line low. The latter method offers the benefit of not requiring a digital to analog converter on each data line. However, one disadvantage of the latter method is that the storage node capacitance within a single pixel is a non-linear function of the voltage when supplied by the gates and junctions of the transistors. Another disadvantage is that the storage node capacitance of each pixel varies among the pixels in an array.
As described above, in order to obtain the desired luminous flux from the OLED 212 of FIG. 2, the voltage on the data line 204 is adjusted to control the current through the drive transistor 208. Unfortunately, current flow through the drive transistor also depends on characteristics of the drive transistor, such as its threshold voltage and transconductance. Large arrays of drive transistors, as required to make a high-resolution display, exhibit variations in threshold voltage and transconductance that often cause the drive currents of the OLEDs to differ for identical control voltages. These variations, in turn, cause a display to appear non-uniform. In addition, different OLEDs emit different intensities of light even when driven with identical currents. Furthermore, the light intensity for a specified drive current drops as an OLED ages and different OLEDs can degrade at different rates, again causing a display to appear non-uniform.
Active matrix pixels are preferably implemented with a silicon substrate instead of a transparent dielectric substrate because transparent dielectric substrates require the transistors to be built as thin film devices. It is difficult to obtain a tight distribution of threshold voltages in large arrays of thin-film transistors, especially as more transistors are needed to make the luminous flux from each pixel insensitive to threshold variations. For this reason, OLED displays are often fabricated on a glass substrate. However, if a silicon substrate is used, addressing, driving, and other circuit functions can be easily integrated, particularly if the substrate and process are compatible with CMOS technology. Although known active matrix pixel technology is compatible with older CMOS technology, OLEDs require higher voltages than dense CMOS can tolerate, while dense CMOS is desirable for the small pixels that are required for high-resolution color displays.
As mentioned above, glass substrates are predominantly used in the fabrication of liquid-crystal and OLED displays. One major disadvantage of glass is that it is fragile. Hence, glass-made displays are not very robust and they tend to break upon impact. A glass substrate is also sensitive to heat, which imposes a limit on the maximum temperature that it can be exposed at during processing. Furthermore, future applications, which may demand some degree of comformability/flexibility in the display, that cannot be readily satisfied with displays fabricated on glass.
Polysilicon (poly-Si) technology has started penetrating the market of smaller displays, such as displays used for personal-digital-assistants (PDAs), cellular phones, car navigation systems, etc. With current technology, thin poly-Si films are typically formed by the deposition of amorphous silicon films and subsequent transformation to polycrystalline silicon upon a suitable thermal treatment. The maximum temperature and the duration of such thermal treatment are constrained by the thermal budget that the substrate can accommodate, before it breaks or it is otherwise damaged.
Glass substrates are generally restricted to processing temperatures below ˜650° C. Higher temperatures, up to approximately 725° C., can possibly be used without damage, but only for very short periods. This low temperature constraint results in poorer quality poly-Si films, which are not compatible with the fabrication of high performance devices and circuits. An alternative approach is to induce the phase transformation is by laser annealing the film, using excimer lasers for example. This process results in very rapid heating of the film, affecting the phase transformation without excessively heating the underlying substrate. Even though the laser annealing process is quite effective, it tends to be more costly than simply heating the film by conventional means.
Another important step in the fabrication of the displays is deposition of gate insulator (GI) layer. In the semiconductor industry, such films are typically formed by thermal oxidation at high temperatures (>1000° C.) to ensure high quality films. However, these high temperatures cannot used when the substrates are made of glass. Hence, a compromise in the quality of the GI layer is typically required for devices made on glass.
It would be advantageous to identify substrates which are more robust than glass, and that permit the fabrication of conformable/flexible displays.
It would also be advantageous if displays, which are built on the above-mentioned substrates, could be made compatible with higher processing temperatures. In that case, higher quality poly-Si material could be obtained, compatible with the fabrication of high performance devices and circuits.